Expression of the immediate early genes of herpes simplex virus I requires the assembly of a multiprotein RNAPII enhancer complex consisting of several viral and cellular transcription factors. This assembly therefore provides a model for the analysis of the components involved in the specific activation of gene expression. The present study focuses upon the characterization of these factors, their interactions, and the mechanisms by which they specifically induce the expression of the viral genes. Although several of these protein factors have been purified and characterized, little is known concerning the mammalian C1 factor, a large protein complex which interacts with the viral transactivator and is required for the enhancer complex assembly. Therefore, this factor was purified from mammalian cells, and cDNAs encoding the polypeptide were isolated. Analysis of this factor indicate that it is a novel transcription factor which is specifically proteolytically processed from a large precursor protein. The continuing studies focus upon questions concerning the tissue distribution of this factor, the specific processing of the precursor protein, and the molecular domains which determine its interaction with the enhancer complex components. Additionally, homologous factors which are present in other animal systems are being investigated to allow for the genetic analysis of these proteins.